ABSTRACT Pulmonary hypertension is associated with poor quality of life, impaired functional tolerance and limitation of physical activity. Despite optimal available therapy for PH, patients report fatigue, decreased functional capacity, and worsening quality of life. Exercise intolerance (i.e. reduced VO2 max) is associated with reduced skeletal muscle (SkM) mitochondrial function and cellular respiration. Recent studies have demonstrated that changes in organization of electron transport chain complexes can significantly alter mitochondrial respiration and energy production. Binding of individual electron transport chain complexes into large molecular weight supercomplexes (SC) increases respiration efficiency, reduces damaging ROS, and improves ATP production. SC can consist of complex I and multiple units of complex III and IV in direct association to allow direct electron transfer. In preliminary studies, we have found that SkM in rats with PH have greatly reduced I/III/IV supercomplex assembly and this is associated with reduced VO2 max determined by maximal treadmill exercise capacity. Our central hypothesis is that reduction in mitochondrial SC in SkM contributes to exercise intolerance in PH, and that increasing SC can alleviate PH-induced SkM dysfunction. We will test this hypothesis using a well-established rat model of pulmonary hypertension that recapitulates the pathophysiological aspects as well as exercise intolerance observed in patients with PH. In Aim 1, we will evaluate PH-induced changes in SkM mitochondrial SC formation and associated changes in respiratory function, mitochondrial content, and cristae architecture in a preclinical PH model and PH patients. In addition we will verify these changes are indeed present in human SkM samples from PH patients. In Aim 2, we will determine molecular mechanisms underlying altered mitochondrial function and SC assembly in isolated SkM fibers and differentiated primary SkM myotubes from control and PH animals Finally in Aim 3, we will determine if increasing mitochondrial supercomplex formation in SkM by exercise or drug therapy results in improved functional capacity (i.e. VO2 max). These will be the first studies to evaluate the role of mitochondrial SC in exercise intolerance associated with chronic medical condition such as PH, and the first studies to directly target SC assembly to alleviate SkM dysfunction.